711 



EMBRYOLOGY. 



different from what we notice in the starfish embryo, where the mouth is 

 formed before the anus has been much bent from its original position.* 



The large accumulations of yolk-masses round the rods r / cannot fail to be 

 noticed in the stages just described (Figs. 41 -4s). This contrasts strikingly 

 with the starfish embryos, in which we find nothing of the kind, and where 

 the body of the young embryo is quite remarkable for its greal transparency, 

 which only increases with age, while in the Echinus embryos the great accu- 

 mulation of yolk-masses renders them somewhat opaque even in their early 

 stasres: the increase of calcareous cells in somewhat more advanced forms (as 

 Figs.:,:, 54) makes it more difficult than in the starfish to trace accurately the 

 minute changes which the rods and water-tubes undergo. We now come 

 to conditions (Figs. 44, 45) which are sufficiently advanced to enable us, by 

 comparing them with still older forms (such as Figs. r,i. :,:. .;;.) to form a cor- 

 rect idea of the mode of transformation from the shape of Figs. 44, 45, to the 

 complicated pluteus represented in Fig. 65- I merely refer to this comparison 



Fig. 45. 



Fir.. 47. 



in a general way, as in the explanation of the different stages it will be 

 carried out more fully, to call attention to the periods which first give us a 

 clew to the development of the different parts, by showing us plainly which 

 portions of the embryo must assume a great prominence, and obtain a more 

 rapid development than others, to pass gradually through the stages which 

 are hereafter figured. In Fig. 44 the difference in the rapidity of the de- 

 velopment of the two water-tubes (w, w) is quite striking. The left water- 

 tube (w) — left when seen from the aboral side, the anal extremity being 

 turned down, as in these figures — is fast pushing through the mass of the 

 embryo, and finds its way to the surface at about the condition represented in 

 Fig. 00, where the water-pore (the madreporic body) allows the water to enter 



* See Fig. 8, Proe. Am. Acad., Vol. VL, quoted above. See PI. I and PI. II., Embryology of Starfish. 



